For a dry human finger, a higher gain is required for a fingerprint unlocking device of related art to obtain a fingerprint image with a high signal-to-noise (S/N) ratio. For a sweaty human finger, a lower gain is required for a finger unlocking device of related art to obtain a fingerprint image with the high S/N ratio. Taken human fingers with different dry to wet levels into consideration, the fingerprint unlocking device needs to adopt different gains to obtain diverse fingerprint images and then to get fingerprint images with a high S/N ratio. However, it takes a lot of time for the fingerprint unlocking device to finish the job, which worsens the user experience.